1. Field of the Invention
This invention relates to an apparatus for recording/reproducing an optical record carrier such as an optical disk which has a photo sensor composed of a plurality of elements which divide a far field image of an information pit on the record carrier, and more particularly to a focusing error detection system which obtains a focusing error signal by measuring a phase difference between outputs of the photo sensor elements.
2. Description of the Prior Art
A typical optical information recording/reproducing apparatus is disclosed in U.S. Pat. No. 4,051,527, issued Sept. 27, 1977. The apparatus derives a focusing error signal from a far field image of an information structure, which is typically composed of a plurality of pits aligned along a spiral track formed on an optical disk, or the record carrier. The apparatus comprises an optical pick-up and a phase detection circuit. The optical pick-up has a laser light source for emitting a laser light beam, an objective lens for concentrating the laser light beam on the record carrier, and a photo sensor divided into two independent elements for converting the back-coming laser light beam from the record carrier to electric signals. The phase detection circuit has a subtractor and an adder for respectively subtracting and adding the electric signals from the photo sensor elements, a phase shifter for delaying the phase of an output of the adder by 90 degrees, a multiplier for multiplying outputs of the subtractor and the phase shifter, and a low-pass filter for smoothing an output of the multiplier.
The photo sensor is attached to the optical pick-up so as to be disposed in the far field of the information structure in order that the sensor elements divide a half plane of the far field image in two. When the information structure on the optical disk is about the focal point of the objective lens of the optical pick-up, the reflected light beam from the optical disk is diffracted to a zero-order beam and higher-order beams. In the far field area, those diffraction beams, overlapping one another, produce bright and dark interference stripes the width of each of which is a function of the focus error. Furthermore, when the focused laser light beam scans the track on the record carrier (when the optical disk is rotating), the stripes also move in the parallel direction of the track image projected on the photo sensor. The direction of the movement of the stripes depends upon whether the focus error is positive or negative. For example, when the focus error is positive, that is, the distance between the information structure and the principal plane of the objective lens is longer than the focal length of the objective lens, the stripes move in the direction of the movement of the record carrier, and in the opposite case, that is, when the distance between the information structure and the principal plane of the objective lens is shorter than the focal length of the objective lens, the stripes move in the opposite direction. Such behavior of the stripes in the far field can be measured as a phase difference between outputs of the two sensor elements, which are aligned in the direction of the track image.
In the apparatus disclosed in U.S. Pat. No. 4,051,527, the phase detection circuit converts the phase difference to a DC signal, i.e., a focus error signal. The adder produces a reference signal S1 by adding the outputs of the photo sensor elements, and the subtractor produces a reference signal S2 by subtracting the output of one of the photo sensor elements from the output of the other. The phase shifter delays the phase of the reference signal S1 by 90 degrees (a quarter cycle of the reference signal S1), hereon the reference signal S1 is to be periodic. Then the multiplier multiplies the delayed reference signal S1 and the reference signal S2. The output of the multiplier is averaged by the low-pass filter.
Provided that such data composed of alternately appearing `1` and `0`, i.e., 10101010101010, are recorded on the record carrier, the output of each photo sensor element is an alternating signal expressed by a sine or cosine function. In this case `1` corresponds to a pit on the record carrier. For example, outputs s1 and s2 of the two photo sensor elements can be expressed as: EQU s1=cos (.omega.t-pd) (1) EQU s2=cos (.omega.t+pd) (2)
where `.omega.` means the angular frequency of the outputs of the photo sensor elements, and `pd` means the phase difference as a function of the focus error. Then the reference signals S1, S2 are: EQU S1=s1-s2=2 sin (.omega.t) sin (pd) (3) EQU S2=s1+s2=2 cos (.omega.t) cos (pd) (4)
Thereafter the phase shifter gives a phase offset of 90 degrees to S2 to obtain the delayed reference signal S2': EQU S2'=2 cos (.omega.t-90) cos (pd)=2 sin (.omega.t) cos (pd) (5)
The output of the multiplier, S3, can be obtained by multiplying (3) and (5): ##EQU1## The low-pass filter cuts the high frequency term to obtain an output signal Sf: EQU Sf=sin (2pd) (7)
This signal Sf is the focus error signal, which is almost in proportion to the phase difference when the phase difference `pd` is small enough.
In the above-described apparatus, however, the sensitivity of the focus error detection decreases in the case that `1` and `0` are not recorded alternately on the record carrier, in other words, the distance between two successive pits (corresponding to `1`) is relatively large. As a typical example, when such data as ---010000000010000000010-- are recorded on the disk, the photo sensor output signals can no more be expressed by a simple sine or cosine function as shown in the equations (1), (2). When being low-pass-filtered, the phase difference signal corresponding to the equation (4) is averaged and becomes almost zero, because the phase difference is obtained only when a transition between `1` and `0` has occurred.
In the case of ROM disks, such as video discs or digital audio discs, such a problem seldom occurs, because the run-length of record data is limited. But when applying this technique to writable/erasable disks, the problem occurs. In general, such a focus detection system as obtaining a focus error signal from the far-field image of recorded data is not suitable to the writable/erasable disks because there is no `recorded data` on them before writing. However, this technique can be utilized for the sampling servo system with a preformatted disk. For example, U.S. Pat. No. 4,562,564 shows that servo marks for tracking servo are provided on the disk in manufacturing process. Those marks, of course, can be used for focus error detection. But, there should be a long interval between two successive marks so as to get enough recording area between those two marks. It means that it is very difficult to derive a focus error signal from a writable/erasable disk by the conventional technique even if servo marks are buried beforehand on the disk.